Radial finned heat sink

ABSTRACT

A radial finned heat sink assembly for an electrical component is constructed from a graphite material, which may be resin impregnated. The assembly includes a base, and a plurality of spaced parallel planar fin members supported by the base. Each fin member includes a planar fin of an anisotropic graphite material having graphite layers aligned primarily with the plane of the fin. In a first construction the fin members include integral core portions. In a second construction the core portions are separately formed so that the graphene layers of the core portion are aligned primarily parallel to the core axis so that heat is efficiently transmitted from the base to the plate members along the core axis.

TECHNICAL FIELD

The present invention relates to a heat sink capable of managing theheat from a heat source like an electronic component. More particularly,the present invention relates to a graphite-based heat sink systemeffective for dissipating the heat generated by an electronic component,wherein the heat sink includes a plurality of spaced parallel planarradial fin members.

BACKGROUND OF THE ART

With the development of more and more sophisticated electroniccomponents, including those capable of increasing processing speeds andhigher frequencies, having smaller size and more complicated powerrequirements, and exhibiting other technological advances, such asmicroprocessors and integrated circuits in electronic and electricalcomponents and systems as well as in other devices such as high poweroptical devices, relatively extreme temperatures can be generated.However, microprocessors, integrated circuits and other sophisticatedelectronic components typically operate efficiently only under a certainrange of threshold temperatures. The excessive heat generated duringoperation of these components can not only harm their own performance,but can also degrade the performance and reliability of the overallsystem and can even cause system failure. The increasingly wide range ofenvironmental conditions, including temperature extremes, in whichelectronic systems are expected to operate, exacerbates the negativeeffects of excessive heat.

With the increased need for heat dissipation from microelectronicdevices, thermal management becomes an increasingly important element ofthe design of electronic products. Both performance reliability and lifeexpectancy of electronic equipment are inversely related to thecomponent temperature of the equipment. For instance, a reduction in theoperating temperature of a device such as a typical siliconsemiconductor can correspond to an exponential increase in thereliability and life expectancy of the device. Therefore, to maximizethe life-span and reliability of a component, controlling the deviceoperating temperature within the limits set by the designers is ofparamount importance.

Heat sinks are components that facilitate heat dissipation from thesurface of a heat source, such as a heat-generating electroniccomponent, to a cooler environment, usually air. In many typicalsituations, heat transfer between the solid surface of the component andthe air is the least efficient within the system, and the solid-airinterface thus represents the greatest barrier for heat dissipation. Aheat sink seeks to increase the heat transfer efficiency between thecomponents and the ambient air primarily by increasing the surface areathat is in direct contact with the air. This allows more heat to bedissipated and thus lowers the device operating temperature. The primarypurpose of a heat sink is to help maintain the device temperature belowthe maximum allowable temperature specified by itsdesigner/manufacturer.

Typically, heat sinks are formed of a metal, especially copper oraluminum, due to the ability of metals like copper to readily absorbheat and transfer it about its entire structure. In many applications,copper heat sinks are formed with fins or other structures to increasethe surface area of the heat sink, with air being forced across orthrough the fins (such as by a fan) to effect heat dissipation from theelectronic component, through the copper heat sink and then to the air.

Limitations exist, however, with the use of metallic heat sinks. Onelimitation relates to the relative isotropy of a metal—that is, thetendency of a metallic structure to distribute heat relatively evenlyabout the structure. The isotropy of a metal means that heat transmittedto a metallic heat sink becomes distributed about the structure ratherthan being directed to the fins where most efficient transfer to the airoccurs. This can reduce the efficiency of heat dissipation using ametallic (e.g., copper) heat sink. Moreover, this relative isotropy isnot readily controlled or varied, and provides no opportunity forpreferentially directing heat.

In addition, the use of copper or aluminum heat sinks can present aproblem because of the weight of the metal, particularly when theheating area is significantly smaller than that of the heat sink. Forinstance, pure copper weighs 8.96 grams per cubic centimeter (g/cc) andpure aluminum weighs 2.70 g/cc (compare with graphite articles, whichtypically weigh less than about 1.8 g/cc). In many applications, severalheat sinks need to be arrayed on, e.g., a circuit board to dissipateheat from a variety of components on the board. If metallic heat sinksare employed, the sheer weight of the metal on the board can increasethe chances of the board cracking or of other equally undesirableeffects, and increases the weight of the component itself.

What is desired, therefore, is a heat sink system effective fordissipating heat from a heat source such as an electronic component. Theheat sink system should advantageously be relatively anisotropic, ascompared to a metal like copper or aluminum and exhibit a relativelyhigh ratio of thermal conductivity to weight. One group of materialssuitable for use in heat sinks are those materials known as graphites.

Graphites are made up of layer planes of hexagonal arrays or networks ofcarbon atoms. These layer planes of hexagonally arranged carbon atomsare substantially flat and are oriented or ordered so as to besubstantially parallel and equidistant to one another. The substantiallyflat, parallel equidistant sheets or layers of carbon atoms, usuallyreferred to as graphene layers or basal planes, are linked or bondedtogether and groups thereof are arranged in crystallites. Highly orderedgraphites consist of crystallites of considerable size: the crystallitesbeing highly aligned or oriented with respect to each other and havingwell ordered carbon layers. In other words, highly ordered graphiteshave a high degree of preferred crystallite orientation. It should benoted that graphites possess anisotropic structures and thus exhibit orpossess many properties that are highly directional e.g. thermal andelectrical conductivity and fluid diffusion.

Briefly, graphites may be characterized as laminated structures ofcarbon, that is, structures consisting of superposed layers or laminaeof carbon atoms joined together by weak van der Waals forces. Inconsidering the graphite structure, two axes or directions are usuallynoted, to wit, the “c” axis or direction and the “a” axes or directions.For simplicity, the “c” axis or direction may be considered as thedirection perpendicular to the carbon layers. The “a” axes or directionsmay be considered as the directions parallel to the carbon layers or thedirections perpendicular to the “c” direction. The graphites suitablefor manufacturing flexible graphite sheets possess a very high degree oforientation.

As noted above, the bonding forces holding the parallel layers of carbonatoms together are only weak van der Waals forces. Natural graphites canbe treated so that the spacing between the superposed carbon layers orlaminae can be appreciably opened up so as to provide a marked expansionin the direction perpendicular to the layers, that is, in the “c”direction, and thus form an expanded or intumesced graphite structure inwhich the laminar character of the carbon layers is substantiallyretained.

Graphite flake which has been greatly expanded and more particularlyexpanded so as to have a final thickness or “c” direction dimensionwhich is as much as about 80 or more times the original “c” directiondimension can be formed without the use of a binder into cohesive orintegrated sheets of expanded graphite, e.g. webs, papers, strips,tapes, foils, mats or the like (typically referred to as “flexiblegraphite”). The formation of graphite particles which have been expandedto have a final thickness or “c” dimension which is as much as about 80times or more the original “c” direction dimension into integratedflexible sheets by compression, without the use of any binding material,is believed to be possible due to the mechanical interlocking, orcohesion, which is achieved between the voluminously expanded graphiteparticles.

In addition to flexibility, the sheet material, as noted above, has alsobeen found to possess a high degree of anisotropy with respect tothermal and electrical conductivity and fluid diffusion, comparable tothe natural graphite starting material due to orientation of theexpanded graphite particles and graphite layers substantially parallelto the opposed faces of the sheet resulting from very high compression,e.g. roll pressing. Sheet material thus produced has excellentflexibility, good strength and a very high degree of orientation.

Briefly, the process of producing flexible, binderless anisotropicgraphite sheet material, e.g. web, paper, strip, tape, foil, mat, or thelike, comprises compressing or compacting under a predetermined load andin the absence of a binder, expanded graphite particles which have a “c”direction dimension which is as much as about 80 or more times that ofthe original particles so as to form a substantially flat, flexible,integrated graphite sheet. The expanded graphite particles thatgenerally are worm-like or vermiform in appearance, once compressed,will maintain the compression set and alignment with the opposed majorsurfaces of the sheet. The density and thickness of the sheet materialcan be varied by controlling the degree of compression. The density ofthe sheet material can be within the range of from about 0.04 g/cc toabout 2.0 g/cc. The flexible graphite sheet material exhibits anappreciable degree of anisotropy due to the alignment of graphiteparticles parallel to the major opposed, parallel surfaces of the sheet,with the degree of anisotropy increasing upon roll pressing of the sheetmaterial to increased density. In roll pressed anisotropic sheetmaterial, the thickness, i.e. the direction perpendicular to theopposed, parallel sheet surfaces comprises the “c” direction and thedirections ranging along the length and width, i.e. along or parallel tothe opposed, major surfaces comprises the “a” directions and thethermal, electrical and fluid diffusion properties of the sheet are verydifferent, by orders of magnitude, for the “c” and “a” directions.

SUMMARY OF THE INVENTION

The present invention provides a heat sink apparatus for an electroniccomponent. The apparatus includes a base constructed from a graphitematerial. It includes a plurality of spaced parallel planar fin memberssupported by the base, each fin member including a planar fin of ananisotropic graphite material having graphene layers aligned primarilywith the plane of the fin, so that each fin exhibits thermalconductivity in directions parallel to the plane of the finsubstantially greater than thermal conductivity perpendicular to theplane of the fin.

The fins are preferably formed by pressing the fins from a sheet ofanisotropic flexible graphite material, preferably resin-impregnatedflexible graphite material. The fins may be pressed with an integralcore portion, in which case the graphene layers of both the fin and thecore are substantially aligned in planes generally parallel to the planeof the fins.

Alternatively, the fin members may be pressed with a circular centeropening in which is received a separately formed core portion. Theseparately formed core portion may be formed such that its graphitelayers are substantially aligned in planes generally perpendicular tothe planes of the fins.

It is an object of the present invention is to provide a heat sinksystem exhibiting a relatively high degree of anisotropy.

Yet another object of the present invention is to provide a heat sinksystem having a relatively high ratio of thermal conductivity to weight.

Still another object of the present invention is to provide a heat sinksystem that can be fabricated so as to locate the heat dissipationsurfaces thereof so as to control and/or maximize the dissipation ofheat from the heat source.

Another object is to provide a radial finned heat sink having a stack ofspaced radially extending fin plates, with the graphite layers of eachfin plate substantially aligned in planes primarily parallel to theplanes of the fin plates.

Yet another object of the present invention is the provision of aconstruction for a radial finned heat sink from graphite material, inwhich the core of the heat sink may be constructed to have the graphitelayers thereof aligned primarily in planes parallel to a core axis andperpendicular to the planes of the fins.

Another object is to provide an economical construction for a radialfinned heat sink constructed of graphite material.

Other and further objects, features, and advantages will be readilyapparent to those skilled in the art, upon a reading of the followingdisclosure when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a radial finned heat sinkconstructed from the components of FIGS. 2 and 3.

FIG. 2 is a sectioned perspective view of a radial fin member with anintegral core portion die pressed from a sheet of graphite material.

FIG. 3 is a sectioned perspective view of a base die pressed from asheet of graphite material.

FIG. 4 is a sectioned perspective view of a first embodiment of a radialfinned heat sink constructed with the components of FIGS. 2 and 3.

FIG. 5 is a sectioned perspective view of a fin member with centralopening die pressed from a sheet of graphite material.

FIG. 6 is a sectioned perspective view of a separate core member formedby isostatically pressing a graphite powder to form a core member havingits graphite layers primarily aligned in planes parallel to a centralaxis of the core.

FIG. 7 is a sectioned perspective view of a base formed by isostaticallypressing a graphite powder to form a base having its graphite layersprimarily aligned in planes parallel to a central axis of the base.

FIG. 8 is a sectioned perspective view of a second embodiment of aradial finned heat sink constructed from the components of FIGS. 5-7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Graphite is a crystalline form of carbon comprising atoms covalentlybonded in flat layered planes with weaker bonds between the planes. Bytreating particles of graphite, such as natural graphite flake, with anintercalant of, e.g. a solution of sulfuric and nitric acid, the crystalstructure of the graphite reacts to form a compound of graphite and theintercalant. The treated particles of graphite are hereafter referred toas “particles of intercalated graphite.” Upon exposure to hightemperature, the intercalant within the graphite decomposes andvolatilizes, causing the particles of intercalated graphite to expand indimension as much as about 80 or more times its original volume in anaccordion-like fashion in the “c” direction, i.e. in the directionperpendicular to the crystalline planes of the graphite. The exfoliatedgraphite particles are vermiform in appearance, and are thereforecommonly referred to as worms. The worms may be compressed together intoflexible sheets that, unlike the original graphite flakes, can be formedand cut into various shapes and provided with small transverse openingsby deforming mechanical impact.

Graphite starting materials suitable for use in the present inventioninclude highly graphitic carbonaceous materials capable of intercalatingorganic and inorganic acids as well as halogens and then expanding whenexposed to heat. These highly graphitic carbonaceous materials mostpreferably have a degree of graphitization of about 1.0. As used in thisdisclosure, the term “degree of graphitization” refers to the value gaccording to the formula: $g = \frac{3.45 - {d(002)}}{0.095}$

where d(002) is the spacing between the graphitic layers of the carbonsin the crystal structure measured in Angstrom units. The spacing dbetween graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous materials include natural graphites from various sources,as well as other carbonaceous materials such as carbons prepared bychemical vapor deposition and the like. Natural graphite is mostpreferred.

The graphite starting materials used in the present invention maycontain non-carbon components so long as the crystal structure of thestarting materials maintains the required degree of graphitization andthey are capable of exfoliation. Generally, any carbon-containingmaterial, the crystal structure of which possesses the required degreeof graphitization and which can be exfoliated, is suitable for use withthe present invention. Such graphite preferably has an ash content ofless than twenty weight percent. More preferably, the graphite employedfor the present invention will have a purity of at least about 94%. Inthe most preferred embodiment, the graphite employed will have a purityof at least about 98%.

A common method for manufacturing graphite sheet is described by Shaneet al. in U.S. Pat. No. 3,404,061, the disclosure of which isincorporated herein by reference. In the typical practice of the Shaneet al. method, natural graphite flakes are intercalated by dispersingthe flakes in a solution containing e.g., a mixture of nitric andsulfuric acid, advantageously at a level of about 20 to about 300 partsby weight of intercalant solution per 100 parts by weight of graphiteflakes (pph). The intercalation solution contains oxidizing and otherintercalating agents known in the art. Examples include those containingoxidizing agents and oxidizing mixtures, such as solutions containingnitric acid, potassium chlorate, chromic acid, potassium permanganate,potassium chromate, potassium dichromate, perchloric acid, and the like,or mixtures, such as for example, concentrated nitric acid and chlorate,chromic acid and phosphoric acid, sulfuric acid and nitric acid, ormixtures of a strong organic acid, e.g. trifluoroacetic acid, and astrong oxidizing agent soluble in the organic acid. Alternatively, anelectric potential can be used to bring about oxidation of the graphite.Chemical species that can be introduced into the graphite crystal usingelectrolytic oxidation include sulfuric acid as well as other acids.

In a preferred embodiment, the intercalating agent is a solution of amixture of sulfuric acid, or sulfuric acid and phosphoric acid, and anoxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. Although less preferred, the intercalation solution maycontain metal halides such as ferric chloride, and ferric chloride mixedwith sulfuric acid, or a halide, such as bromine as a solution ofbromine and sulfuric acid or bromine in an organic solvent.

The quantity of intercalation solution may range from about 20 to about150 pph and more typically about 50 to about 120 pph. After the flakesare intercalated, any excess solution is drained from the flakes and theflakes are water-washed. Alternatively, the quantity of theintercalation solution may be limited to between about 10 and about 50pph, which permits the washing step to be eliminated as taught anddescribed in U.S. Pat. No. 4,895,713, the disclosure of which is alsoherein incorporated by reference.

The particles of graphite flake treated with intercalation solution canoptionally be contacted, e.g. by blending, with a reducing organic agentselected from alcohols, sugars, aldehydes and esters which are reactivewith the surface film of oxidizing intercalating solution attemperatures in the range of 25° C. and 125° C. Suitable specificorganic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol,decylalcohol, 1, 10 decanediol, decylaldehyde, 1-propanol, 1,3propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose,lactose, sucrose, potato starch, ethylene glycol monostearate,diethylene glycol dibenzoate, propylene glycol monostearate, glycerolmonostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethylformate, ascorbic acid and lignin-derived compounds, such as sodiumlignosulfate. The amount of organic reducing agent is suitably fromabout 0.5 to 4% by weight of the particles of graphite flake.

The use of an expansion aid applied prior to, during or immediatelyafter intercalation can also provide improvements. Among theseimprovements can be reduced exfoliation temperature and increasedexpanded volume (also referred to as “worm volume”). An expansion aid inthis context will advantageously be an organic material sufficientlysoluble in the intercalation solution to achieve an improvement inexpansion. More narrowly, organic materials of this type that containcarbon, hydrogen and oxygen, preferably exclusively, may be employed.Carboxylic acids have been found especially effective. A suitablecarboxylic acid useful as the expansion aid can be selected fromaromatic, aliphatic or cycloaliphatic, straight chain or branched chain,saturated and unsaturated monocarboxylic acids, dicarboxylic acids andpolycarboxylic acids which have at least 1 carbon atom, and preferablyup to about 15 carbon atoms, which is soluble in the intercalationsolution in amounts effective to provide a measurable improvement of oneor more aspects of exfoliation. Suitable organic solvents can beemployed to improve solubility of an organic expansion aid in theintercalation solution.

Representative examples of saturated aliphatic carboxylic acids areacids such as those of the formula H(CH₂)_(n)COOH wherein n is a numberof from 0 to about 5, including formic, acetic, propionic, butyric,pentanoic, hexanoic, and the like. In place of the carboxylic acids, theanhydrides or reactive carboxylic acid derivatives such as alkyl esterscan also be employed. Representative of alkyl esters are methyl formateand ethyl formate. Sulfuric acid, nitric acid and other known aqueousintercalants have the ability to decompose formic acid, ultimately towater and carbon dioxide. Because of this, formic acid and othersensitive expansion aids are advantageously contacted with the graphiteflake prior to immersion of the flake in aqueous intercalant.Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids iscitric acid.

The intercalation solution will be aqueous and will preferably containan amount of expansion aid of from about 1 to 10%, the amount beingeffective to enhance exfoliation. In the embodiment wherein theexpansion aid is contacted with the graphite flake prior to or afterimmersing in the aqueous intercalation solution, the expansion aid canbe admixed with the graphite by suitable means, such as a V-blender,typically in an amount of from about 0.2% to about 10% by weight of thegraphite flake.

After intercalating the graphite flake, and following the blending ofthe intercalant coated intercalated graphite flake with the organicreducing agent, the blend is exposed to temperatures in the range of 25°to 125° C. to promote reaction of the reducing agent and intercalantcoating. The heating period is up to about 20 hours, with shorterheating periods, e.g., at least about 10 minutes, for highertemperatures in the above-noted range. Times of one half hour or less,e.g., on the order of 10 to 25 minutes, can be employed at the highertemperatures.

The thus treated particles of graphite are sometimes referred to as“particles of intercalated graphite.” Upon exposure to high temperature,e.g. temperatures of at least about 160° C. and especially about 700° C.to 1000° C. and higher, the particles of intercalated graphite expand asmuch as about 80 to 1000 or more times their original volume in anaccordion-like fashion in the c-direction, i.e. in the directionperpendicular to the crystalline planes of the constituent graphiteparticles. The expanded, i.e. exfoliated, graphite particles arevermiform in appearance, and are therefore commonly referred to asworms. The worms may be compressed together into flexible sheets that,unlike the original graphite flakes, can be formed and cut into variousshapes and provided with small transverse openings by deformingmechanical impact as hereinafter described.

Flexible graphite sheet and foil are coherent, with good handlingstrength, and are suitably compressed, e.g. by roll-pressing, to athickness of about 0.075 mm to 3.75 mm and a typical density of about0.1 to 1.5 grams per cubic centimeter (g/cc). From about 1.5-30% byweight of ceramic additives can be blended with the intercalatedgraphite flakes as described in U.S. Pat. No. 5,902,762 (which isincorporated herein by reference) to provide enhanced resin impregnationin the final flexible graphite product. The additives include ceramicfiber particles having a length of about 0.15 to 1.5 millimeters. Thewidth of the particles is suitably from about 0.04 to 0.004 mm. Theceramic fiber particles are non-reactive and non-adhering to graphiteand are stable at temperatures up to about 1100° C., preferably about1400° C. or higher. Suitable ceramic fiber particles are formed ofmacerated quartz glass fibers, carbon and graphite fibers, zirconia,boron nitride, silicon carbide and magnesia fibers, naturally occurringmineral fibers such as calcium metasilicate fibers, calcium aluminumsilicate fibers, aluminum oxide fibers and the like.

The flexible graphite sheet can also, at times, be advantageouslytreated with resin and the absorbed resin, after curing, enhances themoisture resistance and handling strength, i.e. stiffness, of theflexible graphite sheet as well as “fixing” the morphology of the sheet.Suitable resin content is preferably at least about 5% by weight, morepreferably about 10 to 35% by weight, and suitably up to about 60% byweight. Resins found especially useful in the practice of the presentinvention include acrylic-, epoxy- and phenolic-based resin systems, ormixtures thereof. Most preferably, however, resin impregnation isminimized, nor does the flexible graphite sheet contain additives suchas ceramic fiber particles, in order to optimize thermal conductivity.

As will be described below, some of the components of the heat sinkapparatus of the present invention may be formed by die pressing thesame from flexible graphite sheet, which may be resin impregnatedflexible graphite sheet. Other components, however, maybe be betterformed by grinding up or comminuting the flexible graphite sheet into apowder, and then pressing that powder into a molded shape.

Once the flexible graphite sheet is prepared, it can then be comminutedby known processes or devices, such as a jet mill, air mill, blender,etc. to produce particles. Preferably, a majority of the particles havea diameter such that they will pass through 20 U.S. mesh; morepreferably a major portion (greater than about 20%, most preferablygreater than about 50%) will not pass through 80 U.S. mesh. It may bedesirable to cool the flexible graphite sheet when it isresin-impregnated as it is being comminuted to avoid heat damage to theresin system during the comminution process.

The size of the comminuted particles should be chosen so as to balancemachinability and formability of the graphite article with the thermalcharacteristics desired. Thus, smaller particles will result in agraphite article which is easier to machine and/or form, whereas largerparticles will result in a graphite article having higher anisotropy,and, therefore, greater in-plane thermal conductivity. Accordingly, theartisan should in most instances employ the largest particles whichpermit forming and machining to the degree necessary.

Once the flexible graphite sheet is comminuted, it is compressed intothe desired shape and then cured (when resin impregnated) in thepreferred embodiment. Alternatively, the sheet can be cured prior tobeing comminuted, although post-comminution cure is preferred.Compression can be by die pressing, roll pressing, isostatic molding orother like compression processes. Interestingly, the isotropy/anisotropyof the final article can be varied by the compression (or molding)pressure, the particular molding process utilized and the size of theparticles. For instance, die pressing will result in greater alignmentof the graphene layers and, thus, a more anistropic final product, thanisostatic molding. Likewise, an increase in molding pressure will alsoresult in an increase in anisotropy. Thus, adjustment of molding processand molding pressure, as well as selection of comminuted particle size,can lead to controllable variations in isotropy/anisotropy. This can beused to control heat conduction of heat sink 10, to most efficientlydissipate heat from electronic component 100. In other words, control ofisotropy/anisotropy can be used to direct heat from electronic component100 to the surfaces of heat sink 10 where heat dissipation is best/mostdesired. Typical molding pressures employed range from under about 7Mega Pascals (MPa) to at least about 240 MPa.

Radial Finned Heat Sink Construction

Radial finned heat sinks are used to cool a heat source such as anelectronic component (like a chip assembly) such as those that areattached to printed circuit boards by ball grid arrays. Heat generatedby the heat source is drawn into the heat sink by conduction and thencarried out of the heat sink by convection through the fins. Multipleradial fins are used and they lie parallel to one another and to thesurface of the heat source. The number of fins, their dimensions andspacing vary depending on cooling requirements. These heat sinks areconventionally made from aluminum alloys such as 6061 Aluminum. Thesealloys have a typical density of about 2.7 g/cm3 and exhibit a thermalconductivity which is isotropic and which is as high as about 221 W/m°C. Machining is required to manufacture these heat sinks and adds totheir cost.

The radial heat sink is mounted above the heat source and can actuallybe attached to the heat source. If mechanical vibrations occur, the massof the heat sink can cause strains that can lead to failures. Thus,reducing the mass of the heat sink will minimize such strains andimprove the reliability of the assembly. A radial heat sink made fromgraphite materials will have a mass substantially less than that of acomparable aluminum heat sink (on the order of 60% or less), which willreduce vibration induced strains.

A first embodiment of a radial fin heat sink of the present invention isshown in perspective view in FIG. 1, and in cross section in FIG. 4, andis identified by the numeral 500. Another embodiment of a radial finnedheat sink in accordance with the present invention is shown incross-section in FIG. 8, and is identified by the numeral 600.

Referring now to FIGS. 1-4, individual one piece fin members 502, shownin FIG. 2, which nest on each other, are pressed in a die. A separateend piece 504 shown in FIG. 3, is also die pressed. As shown in FIG. 4,any number of individual fin members 502 a, 502 b, 502 c, etc. are thenstacked together on top of the end piece 504. A sheet 506 of graphitecan also be placed on the end of the end piece 504 to act as a thermalinterface 506 to the chip itself. If the graphite article is not resinimpregnated, a thin layer of a resin, such as phenolic, acrylic orepoxy-based resin system can be interposed between fin members 502 a,502 b, 502 c and end piece 504.

This assembly is clamped together under pressure and the epoxy resin iscured. The individual fin members 502, end piece 504 and thermalinterface 506 then become bonded together. The end piece 504 may bemachined as necessary (external threads may be cut into it for example)to mechanically attach this assembly to the heat source 550 in whatevermanner is desired.

The radial finned heat sink assembly 500 can generally be described asincluding a base 504 constructed from graphite material, and a pluralityof spaced parallel planar fin members 502 supported by the base 504.Each fin member 502 including a planar fin 510 of an anisotropicgraphite material having graphene layers 508 aligned primarily with theplane of the fin 510, so that each fin 510 has a thermal conductivity indirections parallel to the plane of the fin, as indicated by arrows 511,substantially greater than a thermal conductivity perpendicular to theplane of the fin.

In FIGS. 1-4, each fin member 502 includes a core portion 503, the coreportion 503 and the fin 510 of each fin member 502 preferably beingintegrally formed from a sheet of anisotropic flexible graphitematerial. The core portion 503 includes a core protrusion 505 on a firstside and a complementary core recess 507 on a second side, so that theplurality of fin members 502 can be stacked with the protrusion 505 ofone fin member received in the recess 507 of an adjacent fin member.

The base 504 includes a recess 509 defined therein complementary to theprotrusions 505 of the fin members. The protrusion 505 of one of the finmembers 502 c adjacent the base 504 is received in the recess 509 of thebase, so that the fin members 502 are stacked upon the base 504, withthe core portions of the fin members defining a heat transfer path fromthe base.

In an alternate embodiment of the present invention, illustrated inFIGS. 5-8, individual radial fin members 602, with central holes 604,shown in FIG. 5, are pressed in a die from a flexible sheet material. Aseparate connector piece 606, shown in FIG. 6, is formed fromisostatically compressed or die pressed flexible sheet material orpowder material. The ends 608 and 610 of the connectors 606 are machinedto allow them to interlock with the fin members 602 and with each other.A separate interlocking end piece or base 612, shown in FIG. 7, is alsoformed from compressed or die pressed flexible sheet material or powdermaterial. The connectors 606 and end piece 612 have graphite layers 614and 616 aligned so that the in-plane, high thermal conductivitydirection is along the axis 618 of the connectors 606 and end piece 612as indicated by arrows 619. This permits good heat transfer from theheat source 550 through the core of the heat sink and into the radialfins 602.

As shown in FIG. 8, any number of individual fins 602 and connectors 606are then stacked together on top of the end piece 612. A sheet 620 ofgraphite can also be placed on the end of the end piece 612 to act as athermal interface to the heat source 550 itself. If the graphite articleis not resin impregnated, a thin layer of a resin, such as phenolic-,acrylic- or epoxy-based resin system can be interposed between thestacked elements. This assembly is clamped together under pressure andthe resin is cured. The individual components and the graphite thermalinterface then become bonded together. The end piece 612 may be machinedas necessary to mechanically attach this assembly to the heat source inwhatever manner is desired.

The heat sink 600 of FIG. 8 can be described as having its base 612formed from a graphite material including graphene layers alignedprimarily perpendicular to the planes of the fins, so that the base 612and the connector members 606 define an anisotropic core having adirection of highest thermal conductivity in a direction perpendicularto the planes of the fins.

The heat sink 600 of FIG. 8 can also be described as a radially finnedheat sink 600 for an electrical component 550, including a core having abase 612 for attachment to the electrical component, the core having acore axis 618, the core being constructed of a first graphite materialhaving graphite layers aligned primarily parallel to the core axis 618,so that heat from the electrical component can be efficiently conductedaway from the electrical component through the core in a directionparallel to the core axis 618. The heat sink 600 includes a plurality ofspaced parallel fin plates 602 attached to and extending radiallyoutward from the core, each fin plate being constructed of a graphitematerial having graphene layers aligned primarily parallel to the planeof the fin plate and perpendicular to the core axis, so that heat fromthe core can be efficiently transmitted radially outward along the planeof each fin plate. The core further includes a plurality of stacked coreconnector pieces 606, each core connector piece being constructed from acompressed graphite or a resin and graphite mixture. The core connectorpieces 606 are stacked together, with one of the fin plates 602sandwiched between each two adjacent connector pieces.

Regardless of which construction is used for the heat sink, depending onthe particular application in which heat sink is to be employed, it mayalso be desirable to provide a protective coating on the heat sink, toprevent graphite from flaking or breaking off and interfering withelectronic component. Although not believed necessary in the vastmajority of circumstances, such protective coatings can includeelectrically conductive coatings, such as nickel plating.

As schematically shown in FIGS. 4 and 8, either heat sink 500 or 600 canbe mounted to an electronic component 550 by conventional means, such asby mounting directly to electronic component, using an adhesive, such asa pressure sensitive or thermally activated adhesive (something whichthe relatively low weight of graphite permits); mounting to a thermalinterface such as 506 or 620, if present, such as by an adhesive; ormounting to the board or other object on which electronic circuit 550 ismounted, provided heat collection surface 506 or 620 of heat sink 500 or600 is operatively connected to an external surface of the electroniccomponent 550 (directly or through thermal interface 506 or 620).Electronic component 550 can comprise any electronic device or componentthat produces sufficient heat to interfere with the operation ofelectronic component 550 or the system of which electronic component 550is an element, if not dissipated. Electronic component 550 can comprisea microprocessor or computer chip, an integrated circuit, controlelectronics for an optical device like a laser or a field-effecttransistor (FET), or components thereof, or other like electronicelement. Electronic component 550 includes at least one surface fromwhich heat radiates and which can be used as a source of heat to bedissipated from electronic component 550. Also, as noted, the base maybe threaded or the like to provide a mechanical connection to theelectronic component 550.

The use of graphite to form heat sink 500 or 600 has many significantadvantages. As discussed, the anisotropic nature of graphite allows thepractitioner to direct the heat from external surface of electroniccomponent 550 to the fins 502. Graphite materials have the furtheradvantage of relatively low density, and thus relatively low weight. Forinstance, articles formed from exfoliated graphite particles generallyhave a density below about 1.3 g/cc. High density natural graphitearticles have a density below about 1.9 g/cc. When compared with thedensity of copper—approximately 8.9 g/cc for pure copper—a graphitearticle of the same approximate size and volume of a copper article willweight significantly less.

The weight advantage of graphite over copper or other metals can beexpressed in terms of their respective thermal conductivity. If oneconsiders thermal conductivity per unit weight (sometimes referred to inthe art as specific thermal conductivity), the exfoliated graphite heatsinks of the present invention have a specific thermal conductivity inthe direction of high conductivity of about 0.134 watts-meter² perkilogram-° C. (Wm²/kg° C.) to about 0.184 Wm²/kg° C. or higher, whereascopper heat sinks have a specific thermal conductivity of about 0.019 toabout 0.051 Wm²/kg° C. (for a specific thermal conductivity of 0.051,the heat sink would have to be formed of pure copper). Thus, per unitweight, graphite heat sinks can be far more effective at heatdissipation from an electronic component, without the disadvantages of“loading” a circuit board or other component with excess weight. Whenthe further advantages provided by the anisotropic nature of graphiteare considered, heat sinks of the present invention are distinctlyadvantageous.

The methods of constructing the heat sinks 500 and 600 can both begenerally described as including steps of:

(a) die pressing a plurality of fin plate members from a sheet ofgraphite material having graphite layers aligned primarily in the planeof the sheet; and

(b) assembling the fin plate members so that each fin plate memberdefines one of a plurality of spaced parallel planar fin platesextending radially outward in a plane normal to a central axis of theassembly.

In the method of manufacturing the heat sink 500, step (a) includes diepressing each fin plate member 502 to include an integral core portionhaving graphene layers aligned primarily in planes perpendicular to thecentral axis of the assembly. The method further includes die pressing abase 504 from a sheet of graphite material, and attaching the base 504to the core portion of one of the fin plate members 502, the base 504including graphene layers aligned primarily in planes perpendicular tothe central axis of the assembly.

In the method of manufacturing the heat sink 600 of FIG. 8, step (a)includes die pressing the fin plate members 602 to have a centralopening therethrough 604, and forming a plurality of core members 606from graphite material, each core member 606 being shaped to be closelyreceived within the central opening 604 of one of the fin plate members602 and having graphene layers aligned primarily in planes parallel tothe central axis 618 of the assembly. The fin plate members 602 and thecore members 606 are assembled so that each core member 606 issandwiched between two fin plate members 602.

Thus it is seen that the apparatus and methods of the present inventionreadily achieve the ends and advantages mentioned, as well as thoseinherent therein. While certain preferred embodiments of the inventionhave been illustrated and described for purposes of the presentdisclosure, numerous changes in parts and steps may be made by thoseskilled in the art, which changes are encompassed within the scope andspirit of the appended claims.

What is claimed is:
 1. A radial finned heat sink assembly for anelectrical component, comprising: a base constructed from graphitematerial; a plurality of spaced parallel planar fin members supported bythe base, each fin member including a planar fin of an anisotropicgraphite material having graphene layers aligned primarily with theplane of the fin, so that each fin has a thermal conductivity indirections parallel to the plane of the fin substantially greater than athermal conductivity perpendicular to the plane of the fin, wherein eachfin member has an opening defined therethrough; and a plurality ofstacked connector members, each one of the connector members beingclosely received through the opening of an associated one of the finmembers, each of the connector members being formed from a graphitematerial including graphene layers aligned primarily in a directionperpendicular to the planes of the fins.
 2. The assembly of claim 1,wherein: each fin member includes a core portion, the core portion andthe fin of each fin member being integrally formed from a sheet ofanisotropic flexible graphite material, the core portion including acore protrusion on a first side and a complementary core recess on asecond side, so that the plurality of fin members can be stacked withthe protrusion of one fin member received in the recess of an adjacentfin member.
 3. The assembly of claim 2, wherein: the base includes arecess defined therein complementary to the protrusions of the finmembers; and the protrusion of one of the fin members adjacent the baseis received in the recess of the base, so that the fin members arestacked upon the base, with the core portions of the fin membersdefining a heat transfer path from the base.
 4. The assembly of claim 2,wherein: the graphite materials from which the base and the fin membersare constructed includes a resin, and the base and the fin members arebonded together by clamping the base and fin members together and curingthe resin.
 5. The assembly of claim 1, wherein: the base is formed froma graphite material including graphite layers aligned primarilyperpendicular to the planes of the fins, so that the base and theconnector members define an anisotropic core having a direction ofhighest thermal conductivity in a direction perpendicular to the planesof the fins.
 6. The assembly of claim 1, wherein: the graphite materialsfrom which the base, the fin members and the connector members areconstructed include a resin, and the base, the fin members and theconnector members are bonded together by clamping the base, the finmembers and connector members together and curing the resin.
 7. Theassembly of claim 1,further comprising: a thermal interface formed froma sheet of anisotropic flexible graphite material attached to the baseon a side thereof opposite from the fin members.
 8. The assembly ofclaim 1, wherein: each of the fin members are die pressed from a sheetof resin impregnated anisotropic flexible graphite material.
 9. Aradially finned heat sink for an electrical component, comprising: acore, including a base for attachment to the electrical component, thecore having a core axis, the core being constructed of a first graphitematerial having graphene layers aligned primarily parallel to the coreaxis, so that heat from the electrical component can be efficientlyconducted away from the electrical component through the core in adirection parallel to the core axis, wherein the core further comprisesa plurality of stacked core connector pieces, each core connector piecebeing constructed from a compressed resin and graphite mixture; and aplurality of spaced parallel fin plates attached to and extendingradially outward from the core, each fin plate being constructed of agraphite material having graphene layers aligned primarily parallel tothe plane of the fin plate and perpendicular to the core axis, so thatheat from the core can be efficiently transmitted radially outward alongthe plane of each fin plate.
 10. The radially finned heat sink of claim9, wherein: the core further includes a plurality of core connectorpieces; and the core connector pieces are stacked together, with one ofthe fin plates sandwiched between each two adjacent connector pieces.11. The radially finned heat sink of claim 10, wherein: the graphitematerials from which the connector pieces, the base and the fin platesare constructed include a mixture of resin and graphite; and the corepieces, the base and the fin plates are bonded together by curing.
 12. Aradial finned heat sink assembly for an electrical component,comprising: a base constructed from graphite material; a plurality ofspaced parallel planar fin members supported by the base, each finmember including a planar fin of an anisotropic graphite material havinggraphene layers aligned primarily with the plane of the fin, so thateach fin has a thermal conductivity in directions parallel to the planeof the fin substantially greater than a thermal conductivityperpendicular to the plane of the fin; and a thermal interface formedfrom a sheet of anisotropic flexible graphite material attached to thebase on a side thereof opposite from the fin members.
 13. The assemblyof claim 12, wherein: each fin member includes a core portion, the coreportion and the fin of each fin member being integrally formed from asheet of anisotropic flexible graphite material, the core portionincluding a core protrusion on a first side and a complementary corerecess on a second side, so that the plurality of fin members can bestacked with the protrusion of one fin member received in the recess ofan adjacent fin member.
 14. The assembly of claim 13, wherein: the baseincludes a recess defined therein complementary to the protrusions ofthe fin members; and the protrusion of one of the fin members adjacentthe base is received in the recess of the base, so that the fin membersare stacked upon the base, with the core portions of the fin membersdefining a heat transfer path from the base.
 15. The assembly of claim13, wherein: the graphite materials from which the base and the finmembers are constructed includes a resin, and the base and the finmembers are bonded together by clamping the base and fin memberstogether and curing the resin.
 16. The assembly of claim 12, furthercomprising: each fin member having an opening defined therethrough; anda plurality of stacked connector members, each one of the connectormembers being closely received through the opening of an associated oneof the fin members, each of the connector members being formed from agraphite material including graphene layers aligned primarily in adirection perpendicular to the planes of the fins.
 17. The assembly ofclaim 16 wherein: the base is formed from a graphite material includinggraphite layers aligned primarily perpendicular to the planes of thefins, so that the base and the connector members define an anisotropiccore having a direction of highest thermal conductivity in a directionperpendicular to the planes of the fins.
 18. The assembly of claim 16,wherein: the graphite materials from which the base, the fin members andthe connector members are constructed include a resin, and the base, thefin members and the connector members are bonded together by clampingthe base, the fin members and connector members together and curing theresin.
 19. The assembly of claim 12, wherein: each of the fin membersare die pressed from a sheet of resin impregnated anisotropic flexiblegraphite material.